US cartoon Phineas and Ferb made its debut on Disney Channel in early 2008, and has since gone on to be one of the channel's most popular series, attracting more than 289m viewers worldwide since that first broadcast.

One of its co-creators, Jeff 'Swampy' Marsh, gave a keynote at the Children's Media Conference in Sheffield today, explaining the creative workflow behind the show, and how it has evolved over time.

Marsh got into animation "pretty late in life" at the age of 28, with his first professional job drawing backgrounds for The Simpsons. Four seasons later, he moved on to the show Rocko's Modern Life, where he got his first chance to write and direct, and also met his Phineas and Ferb co-creator Dan Povenmire.

He went on to work on King of the Hill and Aaagh! It's the Mr Hell Show – with the late Bob Monkhouse – before reconvening with Povenmire to start work on their joint project.

"Phineas and Ferb has been massively successful. It's kinda scary and humbling," he said. "There is no way that we imagined this show would reach so many people and so many viewers. We were pretty cocky and thought we'd get a couple of seasons out of it, but what's happened with it has been staggering."

The idea was born in 1993, but the call from Disney didn't come until 13 years later. "It was the rantings of two middle-aged men who believed kids didn't get out and do things any more," he said.

"All we saw kids doing these days was watching DVDs and playing video games. So that was the genesis of Phineas and Ferb: it was kids who got out and did stuff."

The series – if you haven't seen it – focuses on two boys "trying to make the best of every summer's day", and the efforts of their sister to spoil their fun, and the secret-agent antics of their pet platypus Perry.

The show nearly got commissioned by Nickelodeon, Cartoon Network and Fox Kids before landing with Disney. "It had always gotten a good response. We'd just never gotten a sale out of it," said Marsh.

"It was always the show we thought we should be making. The one where when we watched new shows come out, we thought 'Why isn't anyone making Phineas and Ferb?'... So we finally got it sold, and started the development process in 2005."

Marsh talked about the process of feedback from the network – "network notes" – on everything from changing characters' hair colours to ditching the actress voicing the mum character because her voice was "too flat".

"We'd both done a lot of television, and a lot of very successful shows, and after a lot of years of pitching, we'd taken the attitude that we're going to do it how we think we should do it. And if it doesn't work, we're going to keep pitching it. So there were a lot of things we stuck to our guns on," he said.

Marsh and Povenmire also didn't do scripts, which Marsh admitted wasn't how Disney liked to work at the time. "With a lot of cajoling, we did get the network to say 'yes, you can do that', which I have to applaud them for," he said.

The pair did give on some things: like the network's desire to have a girl in the show who'd be "aspirational" – which is where they beefed up the role of the Isabella character.

Marsh added that he was keen never to underestimate their viewers. "We decided very early on that children are much more intelligent than people give them credit for. So we never took a joke out because we thought it was too clever for them, or it would go over their heads," he said.

More from the creative philosophy behind the show: the decision to avoid "populating it with jerks and idiots" just to get laughs. "It makes it much harder to do the comedy," he admitted. "It's really a lot harder to do, but it's more rewarding when you get it right."

The story and the characters come before the gags, too. "Nothing we do in Phineas and Ferb is a gag tree," he said. "If the audience doesn't care about the characters, they're not going to care about your gags. That's been a frustration for some of our gaggier writers, but it's worked out well."

Marsh was asked about opportunities to tell stories on other, digital platforms: YouTube and mobile/tablet apps.

"It's great: the nice thing now is that people have the ability to create fully-formed visions of what they want. The tools are out there – sometimes downloadable for free! – where you can make your own show. It's thrilling," he said.

"We used to literally edit Super 8 videos to make a movie. And they turned out awful. So it's terrific, the fact that there are now venues for different types of programming, and they can be delivered in different types of ways. It's thrilling."

Marsh also talked about British influences on his work, from Danger Mouse and Count Duckula to Lenny Henry on Tiswas. "I remember Lenny Henry sitting in a bath of milk, I believe, and just thinking 'what is going on here?'. It was something for everybody, and I always thought that was really cool: something that could be enjoyed by adults and teenagers and kids too."

How long will Phineas and Ferb keep going? "As long as they'll have us," said Marsh. "We don't know what we're going to do after that... Hopefully one day we'll get to do a musical. We'll see. Right now we're getting our head down and making the show as fast as we can."

Have the pair ever considered doing a show with girl leads rather than boys? One of the reasons Disney picked the show up originally was its desire to attract more boys to watch the Disney Channel "without alienating the girls".

Marsh said that the show has never been targeted at boys or girls specifically, though. "I would resist anybody saying there is a boy or a girl's show. If you're doing comedy, funny is funny," he said. "It would be the same thing that we do for kids: we don't make a show for kids. We just try not to alienate kids."

But the phrase that rang in the memory from Marsh's keynote was his distinction between Phineas and Ferb's target audience – children – and the real target audience in his and Povenmire's minds when working on the show.

"We've never made this show for kids. We've always made it for each other."